1. Field of the Invention
The present invention relates to a method for creating a color conversion definition (a look-up table or the like) for image output devices, and an image processing apparatus implementing the method for creating a color conversion definition.
2. Description of the Related Art
In consideration of color engineering, an illumination light illuminates an object, and a reflected light or a transmitted light enters human eyes to be recognized as a color of the object. As shown in FIG. 12, if a spectroscopy spectrum of an illumination light is P (λ), and a spectral reflectance of an object is R (λ), a spectral distribution φ (λ) of a reflected light which enters the human eyes from the object is expressed by φ(λ)=R (λ)P (λ).
Spectroscopy spectrum P (λ) of an illumination light, spectral reflectance R(λ) of an object, and spectral distribution φ(λ) of a reflected light from the object are graphed in FIG. 13. The abscissas denote wavelengths (nm), and the ordinates denote radiant energies or reflectance factors.
A light φ(λ) entering an eye is converted into a signal through cones which are visual cells having photosensitivity on the retina, and the signal is transmitted to the brain to be recognized as a color. In consideration of color engineering, a response from the cones is handled as a CIEXYZ color coordinate system (hereinafter abbreviated as XYZ) If a spectral distribution of a reflected light is φ(λ), and color-matching functions are x (λ), y (λ), and z (λ), tristimulus values X, Y, and Z are expressed by:X=k∫φ(λ)x(λ)dλY=k∫φ(λ)y(λ)dλZ=k∫φ(λ)z(λ)dλ  [Expression 1]
When the values of X, Y, and Z are the same, it is considered that a color does not vary even when a spectral distribution φ(λ) of a light entering an eye is different. This characteristic is utilized in image output devices such as printers and displays. In a printer, inks (cyan/magenta/yellow/black, and the like) are controlled such that a color supposed to be outputted and a color of an output result have the same values of X, Y, and Z under a given illumination light. Provided that X, Y, and Z having same values as values of X, Y, and Z of an input color are outputted, a printer is defined as a printer performing accurate output color reproduction. Further, provided that values of X, Y, and Z added with human favorability are calculated as values of X, Y, and Z of an input color, and the values of X, Y, and Z are outputted, a printer is defined as a printer performing favorable output color reproduction.
In a printer, generally, color appearance of an output result (output color reproduction) is determined by using a D50 illumination light serving as a supplemental illumination light by the CIE, as an illumination light. Output color reproduction is determined by making correspondence between an input color space and a device color space (RGB or CMYK) for an image output device, i.e., by making output color conversion definition. In order to carry out making correspondence of a device color space for an image output device to a color space, it is necessary to know the characteristic of the device color space.
In a case of a printer, a representative color (i.e. a color patch) as a device color of an image output device is actually outputted, and the color is measured, which makes it possible to know the characteristic of the device color space. As an algorithm for creating an output color conversion definition on the basis of the characteristic, a variety of algorithms have been conceived of by a number of manufacturers. Further, some manufacturers provide applications and modules by which it is possible to create an output color conversion definition suitable for the image output device when measured color values of a device color of an image output device are inputted, and users can utilize those.
It is assumed that objects with different reflectance factors or transmission factors have same values of X, Y, and Z under a given illumination light. However, in a case in which the illumination light is changed, those are not necessarily the same values of X, Y, and Z. In that case, when the illumination light is changed, colors of the object appear differently. That is, even if it is designated a color supposed to be outputted and a color of an output result for an image output device are made to have same values of X, Y, and Z under a given illumination light, when the illumination light is different, the color supposed to be outputted and the color of the output result do not necessarily have the same values of X, Y, and Z. That is, in an image output device such as a printer, because color appearance of an output result (output color reproduction) varies in accordance with an illumination light, it is necessary to consider output color reproduction in consideration of an illumination light to observe an output result.
A case in which different illumination lights are utilized as an illumination light in an observational environment for an output result by respective users such that a user A observes it under an illumination light A, a user B observes it under an illumination light B, and a user C observes it under an illumination light C, is conceivable. Further, even when a user is the same person, a case in which an observational environment for an output result of an image output device varies depending on periods of time such that it is under an illumination light A (sunlight) during daytime hours, and it is under an illumination light B (interior illumination) during nighttime hours, and a case in which an observational environment for an output result of an image output device varies depending on places for observation such that it is under an illumination light A at a place A, and it is under an illumination light B at a place B, are conceivable. It is necessary to consider output color reproduction under a plurality of illumination lights with respect to output color reproduction in an image output device.
In order to accurately perform output color reproduction in consideration of illumination lights, there is a method for realizing separate output color reproductions based on the illumination lights for respective illumination lights. This can be realized such that color conversion definitions to realize separate output color reproductions are prepared in advance for respective illumination lights, and a color conversion definition to realize output color reproduction under an arbitrary illumination light is determined on the basis of color conversion definitions to realize output color reproductions with respect to illumination lights prepared in advance. Further, it is possible to determine a color conversion definition dynamically in accordance with observational environments.
Further, it has been examined to create one color conversion definition by which it is possible to perform optimum output color reproduction in which an uncomfortable feeling in appearance under each illumination light is made less in consideration of an appearance of an output result under the illumination lights in a plurality of observational environments. In a technology disclosed in Japanese Patent Laid-Open No. 2003-283851, a color is measured under an illumination light A, and an output color conversion definition to optimize an output result under the illumination light A is created. Then, it has been disclosed in this document that a color of an output result by the created color conversion definition is measured under illumination lights A, B, and C, and the created output color conversion definition is modified such that a mean value or a centroid value of actual colors under respective illumination lights appears as a color supposed to be outputted. Further, in Japanese Patent Laid-Open No. 2003-153015, it has been disclosed that output color reproductions under the illumination lights A, B, and C are respectively determined, and an output color conversion definition is determined such that errors in those output color reproductions are made minimum.
As described above, it is preferable to take output color reproductions under a plurality of illumination lights into consideration with respect to output color reproduction of an image output device. That is, it is necessary to make the same output result appear without any uncomfortable feeling when a same output result is observed under different illumination lights.
As a conventional art, there has been a technology having an aim to optimize an output result in the observational environment in accordance with an illumination light in an observational environment of each user. Because it is an aim to optimize an output result in the observational environment in each observational environment, it is necessary to change an output result by changing a color conversion definition when an observational environment is changed. In an image output device such as a display, because the modification is easy, it suffices to change an output result. However, with respect to an output result printed by an image output device such as a printer, there is no method but a method of outputting again when an observational environment is changed.
There are many cases in which one printing object is viewed in different observational environments, it is necessary to output it in consideration of illumination lights in a plurality of observational environments. In order to solve these problems, it is necessary to create one color conversion definition by which it is possible to perform optimum output color reproduction in which an uncomfortable feeling in appearance of an output result under illumination lights in a plurality of observational environments, and an uncomfortable feeling in appearance under the respective illuminations are made less.
In the technology disclosed in Japanese Patent Laid-Open No. 2003-283851, a color is measured under a given illumination light A, and an output color conversion definition to optimize an output result under the illumination light A is created, and colors of the output result by the created color conversion definition are measured under illumination lights A, B, and C, and the color conversion definition is modified such that the output result have a mean value or a centroid value of the measured color values. Because it takes time for color measuring operation, it takes more time for measuring colors twice before and after creating the color conversion definition. Further, with respect to a mean value or a centroid value, all observational environments to be assumed are handled equally, it cannot be considered that this is necessarily favorable in all use cases.
For example, in a case in which there are three observational environments, supposing that a frequency in use is 50% in an observational environment A, 30% in an observational environment B, and 20% in an observational environment C, weighting must be applied to the observational environment A, and it is impossible for the conventional art to respond thereto. Briefly, it is preferable to realize optimum output color reproduction in accordance with a plurality of observational environments in which users actually use the image output device. However, in the conventional art, there has been no proposal or mention of a method for realizing that.
In the technology disclosed in Japanese Patent Laid-Open No. 2003-153015, it is necessary to perform a computation for an output result for each illumination light in a plurality of observational environments in order to minimize errors once, and the number of process for calculation and the number of processes for creation are many. Further, because a color conversion definition is created so as to minimize errors using a plurality of values of an output result, it is necessary to apply a unique algorithm for creating a color conversion definition, and other algorithms for creation cannot be applied thereto. In consideration of general versatility, the other algorithms are preferably applicable to such applications and modules by which it is possible to create a color conversion definition by using measured color values of a device color as input values in the conventional art.
In a printer, an output is carried out by controlling a plurality of color materials (cyan ink/magenta ink/yellow ink/black ink, and the like). Because reflectance factors of the respective color materials are different from one another, the extent of color shift of every illumination is different in every color material. That is, even in an output of the same image output device, because a combination of color materials differs depending on colors, an extent of color shift of every illumination is different depending on colors. In the conventional art, color correction added with this characteristic has not been carried out.